Process for the preparation of glass fiber optical waveguides with increased tensile strength

ABSTRACT

For the production of glass fiber optical waveguides with increased tensile strength, these are drawn from a glass preform with a single-layer or multi-layer of additional sheathing of a glass material, with the material of at least the outermost layer in each case having a lower coefficient of thermal expansion as compared with the layer located underneath it or with the material of the preform. During the drawing process, the fiber is drawn from the preform in the cold state with increased tensile force of 70 to 200 cN.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the preparation of glassfiber optical waveguides with increased tensile strength by drawing thewaveguides from a glass preform with a single-layer or multi-layeradditional sheathing of a glass material, with the material of at leastthe outermost layer in each case having a lower coefficient of thermalexpansion as compared with the layer underneath it or with the materialof the preform.

2. Description of the Prior Art

Processes of the above-mentioned type are known from DE OS 24 19 76. Inthe case of optical fibers consisting of a high-refraction core andlow-refraction cladding, to improve the mechanical properties, there hasalready (DE OS 24 19 786) been arranged, over the cladding, a sheathwhose coefficient of thermal expansion is lower than that of thecladding and/or lower than that of the combination of the core andcladding. In this manner, in the production of the fiber, a compressivestress is built up in the sheath, which makes the fiber insensitive totensile and flexural stress.

However, a process is also already known from DE-OS 27 27 054, in which,in order to increase the tensile strength of a glass fiber, there hasbeen additionally applied to the cladding of a fiber preform at leastone protective layer of a glass material, which has a lower coefficientof thermal expansion as compared with the adjoining glass material ofthe fiber preform or fiber. In order to achieve this, a glass based onsilicon dioxide which is doped with one or more oxides of the chemicalelements lithium, sodium, magnesium, calcium, boron and others is usedfor the protective layer. The glass preform provided with a protectivelayer of this type is drawn to a fiber in a fiber drawing machine at theusual drawing temperatures of approximately 2000° C.

In another known glass fiber described in DE OS 28-26 010, in order toincrease the tensile strength, the outer sheath consists of at least onelayer which, because of its low coefficient of thermal expansion ascompared with the cladding, exerts pressure on the fiber-opticalstructure. Because, in this case, the difference between the twocoefficients of thermal expansion should be as large as possible, metallayers are also used to achieve the desired effect, with particularconsideration given to the use of aluminum and tin.

However, the mechanical properties of glass fiber optical waveguidesprepared in this manner do not meet the requirements which result whenoptical waveguides of this type, spooled in unwindable lengths, are usedfor the remote control of instruments and systems.

SUMMARY OF THE INVENTION

Starting from the prior art described, the invention is therefore basedon the object of finding possibilities leading to an increase inbreaking strength, improvement in aging stability, and immobilization ofmicro-cracks.

According to the invention, this object is achieved by the fact that thefiber is drawn cold from the preform with increased tensile force. Theinvention is based on the finding that the measures taken so far, namelythe production of compressive stresses by means of suitable outersheaths and drawing of the fiber at temperatures above 2000° C., doindeed lead to an increase in tensile strength, but the values achievedin this way are not sufficient if special requirements are to be appliedto the fibers. If, however, as provided for in the invention, the fiberis drawn from the preform in the cold state, very high compressivestresses are produced in the peripheral region of the fiber, which aremany times higher than the compressive stresses of the preform. The coldstate is, for example, a temperature range close to the particular glassmelting point, in which case the tensile force required for this purposeis set to a high level. During the drawing process, therefore, the innerregion of the multi-layer preform structure always shows a higherviscosity than the outer region which serves as a sheathing. This, ofcourse, applies only to the site at which the stresses are generatedwithin the so-called drawing bulb at the heated end of the preform,formed by the drawing process. The tensile stress applied to the fiberfrom the outside is thus frozen in the interior region with decreasingglass temperature, while the cross-section of the outer layers or of theoutermost sheathing still has a viscosity that is too low for theimpression of forces.

Fibers prepared from a glass preform with an outer layer or an outersheath of a material with a lower coefficient of thermal expansion ascompared with the material of the preform therefore has applied to it atensile force of 70-200 cN, preferably 90-150 cN, during the drawingprocess. Higher tensile stresses lead to higher compressive stresses inthe peripheral region of the fiber and, thus, to higher tensile stressesin the remaining cross-section of the fiber. As a result of thecompressive stresses on the periphery of the fiber, the sizedistribution of micro-cracks at the surface of the fiber is at a lowlevel, as a result the fibers according to the invention have anincreased breaking strength.

As explained in the prior art cited, the additional sheathing appliedover the preform cladding consists of the same base material as thepreform itself, with this base glass material being doped with anydesired element in order to achieve a lower coefficient of thermalexpansion. In contrast to this, the invention, as an additional idea,provides that a synthetic quartz glass be used as a material for thelayer or layers of the glass material with a lower coefficient ofthermal expansion. The measures taken according to the invention, namelycold drawing at extremely high tensile stress values in conjunction withthe synthetic quartz glass, lead to particularly high-grade fibers,which meet mechanical requirements even when such fibers are wound inlong lengths and are used for the control of instruments or systems.

The synthetic quartz glass can, for example, be doped with chlorine,which contributes to the different expansion behavior of the syntheticas compared with the natural glass material, which is the cause forcompressive stresses at the periphery of the preform, which can also bemeasured in the fiber itself.

By doping the glass materials used with foreign substances, thecoefficients of thermal expansion are adjustable over a wide range, butthe difference between the reciprocal expansion coefficients should beat least 4-6 percent in order to ensure that the compressive stresses atthe fiber surface which are a prerequisite for the desired tensilestrengths in the fiber are present during the drawing process.

Furthermore, the advantage of a fiber that is under high compression inthe peripheral region is to be seen not only in higher strength of thefiber itself as compared with known fiber types, but also in the factthat the drawing furnace used for manufacture can be run in a morecost-effective manner, for example, because of the lower drawingtemperatures required. This feature is in stark contrast to themanufacturing process for fibers known from the prior art in which, asalso stated there, the fibers must be drawn at high temperatures inorder to keep the micro-cracks on the glass surface small.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of the waveguide of the present invention.

FIG. 2 is a graph illustrating the stresses in the layers of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The exemplifying embodiment relates to a glass fiber optical waveguide,drawn from a preform in accordance with the characteristics of theinvention. The preform is produced by internal coating of a substratetube and subsequent collapse of this tube. A so-called sleeving tube,made of a material with a lower coefficient of thermal expansion ascompared with the substrate tube, is then slid over the collapsedsubstrate tube. In the present exemplifying embodiment, a syntheticquartz glass doped with chlorine was used as the material for thesleeving tube.

The aforesaid construction of a fiber according to the invention, forexample, with a fiber cross-section of 125 um, is shown in FIG. 1. Inthis figure, in the interior of the fiber is the optical core, overwhich the individual layers deposited from the gas phase are arranged toform the optical cladding. Above this is arranged the substrate tube,which, initially coated both with the core and with the claddingmaterial, has been collapsed to form the preform. The fiber surface isfinally formed by means of the layer of material with a lowercoefficient of thermal expansion, as compared with the substrate tube,used in accordance with the invention, which in the present case is theabove-mentioned synthetic glass. The layer of material with a lowercoefficient of thermal expansion can be applied as a sleeving tube orapplied as a powder which is subsequently sintered.

FIG. 2 shows the change in fiber stress over the whole fibercross-section, with reference to the individual layers in the fibercross-section formed by the core, the cladding, the substrate tube andthe sheathing formed of a sleeving tube. As already stated, theessential feature of the process according to the invention is to beseen in the fact that the fiber is drawn from the preform in the coldstate with increased tensile force, for example, between 90 and 150 cN.As can be seen from FIG. 2, these measures, in contrast to knownprocesses, create high compressive stresses in the region of the fibersurface, which are many times greater than the compressive stresses ofthe preform. The high compressive stresses in the peripheral region ofthe fiber result from the low softening temperature of the material ofthe sleeving tube as compared with the higher softening temperature ofthe optical core, the optical cladding, and the substrate tube.

As a result of the compressive stresses in the peripheral region of thefiber, which can be seen from FIG. 2, the micro-crack size distributionat the fiber surface is at a very low level, and the fibers thus have ahigh breaking strength. If, on the other hand, as has previously beendone with the known processes, a conventional fiber is produced bydrawing from a preform, then this fiber, with impressed tensile stressesin the peripheral region (as is shown by the broken line of the fiberstress diagram in FIG. 2 in the region of the sleeving tube) has astrength in the peripheral region which decreases as the tensile forceduring the drawing process increases. The breaking strength measured fora fiber is increased in proportion to the impressed compressivestrength, since it counteracts the tensile stress applied from outside.If, on the other hand, as has been conventional in the past, tensilestresses are impressed in the peripheral region, i.e., in the region ofthe sleeving tube, then these tensile stresses in the peripheral regionare further increased in proportion to tensile stresses applied from theoutside.

WHAT IS CLAIMED IS:
 1. A process for preparing a glass fiber opticalwaveguide with increased tensile strength comprising the steps of:A.providing a glass preform with a core and an outermost sheathing layerof a glass material, the glass material of the outermost layer has alower coefficient of thermal expansion as compared with materialunderneath the outermost layer; B. heating at least a portion of theglass preform to a temperature close to a melting temperature of theglass material; and C. drawing a glass fiber optical waveguide from theheated portion of the preform using an increased tensile force in therange of 70 to 200 cN.
 2. A process according to claim 1, wherein thetensile force used during the drawing step has a value between 90 and150 cN.
 3. A process according to claim 1, wherein the tensile forceduring the drawing step has a value between 70 and 100 cN.
 4. A processaccording to claim 1, wherein a synthetic quartz glass is used as thematerial for the outermost layer of glass material with a lowercoefficient of thermal expansion.
 5. A process according to claim 4,wherein the synthetic quartz glass is doped with a doping component. 6.A process according to claim 5, wherein chlorine is used as the dopingcomponent.
 7. A process according to claim 1, wherein the core of thepreform comprises an internally coated and collapsed glass tube that iscovered by the outermost layer with the difference between reciprocalcoefficients of thermal expansion of the glass tube and the outermostlayer being at least 4 to 6 percent.
 8. A process according to claim 1,wherein the outermost layer with the lower coefficient of thermalexpansion is a pre-fabricated sleeving tube.
 9. A process according toclaim 1, wherein the outermost layer with the lower coefficient ofthermal expansion comprises a glass material applied onto the preform inpowder form and then sintered.
 10. A process according to claim 1,wherein a synthetic quartz glass is used as the material for theoutermost layer with the lower coefficient of thermal expansion.
 11. Aprocess according to claim 10, wherein the synthetic quartz glass isdoped with a doping component.
 12. A process according to claim 11,wherein chlorine is used as the doping component.